UN Secretary General Antonio Guterres on Monday expressed his concern on the number of people killed in Gaza.

Speaking at a joint press conference with Chancellor of Austria Sebastian Kurz following a meeting in Vienna, Guterres said: “I am particularly worried today with the news about what's happening in Gaza, already with a high number of people killed.”

At least 43 Palestinians were killed and hundreds injured on Monday by Israeli gunfire during anti-occupation rallies in the Gaza Strip, according to the country’s Health Ministry.

Thousands of Palestinians have gathered on Gaza Strip’s eastern border since early morning to take part in protests aimed to commemorate the Nakba anniversary and protest relocation of the U.S. Embassy from Tel Aviv to Jerusalem.

The rallies will culminate on Tuesday, May 15, the 70th anniversary of Israel's establishment -- an event Palestinians refer to as the "Nakba" or "the Catastrophe".